1. Field of the Invention
The present invention relates to a variable valve timing system, and more particularly to a variable valve timing system for an engine of a vehicle.
2. Background of the Related Art
A conventional variable valve timing system 500 is disclosed in Japanese Patent Publication No. 62(1987)-3111, and is shown in FIG. 5. The variable valve timing system 500 is used for an engine of a vehicle (not shown). In the variable valve timing system 500, a timing pulley 501 has an inner helical gear 501a and an outer gear 501b. The outer gear 501b is geared with a timing belt 502. A camshaft 503 is rotatably supported in a cylinder-head 506 of the engine, and has an oil conduit 503a formed inside thereof. A cylindrical member 504 forms an outer helical gear 504a and is held on the cam-shaft 503 by a hollow bolt 507.
A cylindrical piston system 505 includes a first piston 505a, a second piston 505b, a plate 505c, a first spring 505d and a second spring 505e. The first piston 505a and the second piston 505b have respective inner helical gears 505a-1 and 505b-1 and respective outer helical gears 505a-2 and 505b-2. The inner helical gears 505a-1 and 505b-1 are geared with the outer helical gear 504a. The outer helical gears 505a-2 and 505b-2 are geared with the inner helical gear 501a.
The first piston 505a is operatively connected with the plate 505c. The first spring 505d is interposed between the first piston 505a and the second piston 505b, so that the first piston 505a, the second piston 505b and the first spring 505d constitute a scissors gear system for decreasing a backlash.
A cam-shaft cover 508 is fixed to the timing pulley 501 by bolts 509. A pressure chamber 510 is formed between the plate 505c and the cam-shaft-cover 508. The pressure chamber 510 is in fluid communication with the oil conduit 503a via the hollow bolt 507.
In the above-mentioned variable valve timing system 500, the timing belt 502 is driven by a crank-shaft of the engine (not shown). Thus, the timing pulley 501 is rotated by the timing belt 502, and the cam-shaft 503 is rotated through the cylindrical piston system 505. The cam-shaft 503 drives some valves of the engine (not shown), so that some valves are opened or closed. A change of the revolution speed of the engine requires a change in the timing by which these valves are opened or closed.
In the pressure chamber 510, there is provided high-pressure oil supplied from an oil tank (not shown) through a control valve (not shown) and the oil conduit 503a. This causes the cylindrical piston system 505 to move in the rightward direction. Therefore, the relative angle between the timing pulley 501 and the cam-shaft 503 is changed by the helical gears between the timing pulley 501 and the cylindrical piston system 505 and between the cylindrical piston system 505 and the cam-shaft 503. Consequently, the timing by which some valves are opened or closed is changed.
In the normal driving of the engine, the cylindrical piston system 505 receives a torque variation from the cam-shaft 503 as the cam lobes sequentially engage and disengage the cam followers, producing rotational vibrations between the timing pulley and the cam-shaft, as a result of which the cylindrical piston system 505 may move in the rightward direction, even though the pressure chamber 510 is not supplied with high pressure oil. Thus, the spring 505e must be strong to avoid such movement.